The Queen Alexandra's Birdwing (アレクサンドラアゲハ, Arekusandora ageha), formerly known as the Birdwing Butterfly prior to New Horizons, is considered the rarest butterfly in the game, with reference to its immense size and rarity in real life. It can be found during the summer, between June and September (from May to September in New Horizons), from 8 a.m. to 5 p.m.. It can be found flying near flowers.

"Though I know this is a type of tiger butterfly, its size is simply monstrous! Are you trying to terrify me with this behemoth? No, you wouldn't do that. Rather shabby of me to suggest you would... Please accept my apologies!"

"Hoo my, what a butterfly... I now see why it's touted as the world's largest butterfly, eh wot? I'd heard the wings of this particular butterfly can extend over a foot...but up close, they look bigger still! If one of these flew into me while I was out flying... I'd likely pass out in horror and plummet on the spot!"

Upon being donated, the butterfly can be found in the room of the bug exhibit with the Rafflesia. The exhibit has this to say about the Birdwing Butterfly:

"The world's largest butterflies, birdwing butterflies, have wings that can measure 30 cm long.The females have longer wings than the males, while the males' wings have a deeper blue color. Though they are extremely popular with collectors, international trading is either limited or fully banned."

"Hoo! The horror! This behemoth butterfly may be called a Queen Alexandra's birdwing...But I call it the Queen of My Nightmares! It is huge! Indeed, the world's hugest. No butterfly can best its foot-long wingspan. The larvae alone grow to more than 4 inches. As if that weren't appalling enough...they're poisonous! Murderous monsters, indeed!"

When donated, the Queen Alexandra's birdwing flies around the butterfly room in the museum's bug exhibit.

The Queen Alexandra's Birdwing (Ornithoptera alexandrae) is considered the largest butterfly in the world, named after Queen Alexandra, wife of King Edward VII of the United Kingdom. It was discovered by Albert Stewart Meek, who was employed by Lord Walter Rothschild (who named it) in 1907, in Papua New Guinea. It was shot down using a shotgun, but its early stages were discovered and bred through to get to this stage. The female birdwing has a wingspan of up to 31cm, while the male (which appears in game) has a smaller wingspan of up to 26cm. The birdwing flies quite high in the rainforest, making it hard to catch, which is the main reason hunters shoot them with shotguns. The birdwing in Animal Crossing also flies higher than usual butterflies, a reference to its flying habits in the real world.

The birdwing is endangered, threatened by habitat loss. Most of Papua New Guinea's rainforests were destroyed during a volcanic eruption, wiping out parts of the birdwing's habitat.